Myocardial infarction or heart attack remains the leading cause of death in our society. Unfortunately, most of us can identify a family member or close friend that has suffered from a myocardial infarction. Until recently many investigators believed that coronary arteries critically blocked with atherosclerotic plaque that subsequently progressed to total occlusion was the primary mechanism for myocardial infarction. Recent evidence from many investigational studies, however, clearly indicates that most infarctions are due to sudden rupture of non-critically stenosed coronary arteries due to sudden plaque rupture. For example, Little and coworkers (Little, W C, Downes, T R, Applegate, R J. The underlying coronary lesion in myocardial infarction: implications for coronary angiography. Clin Cardiol 1991; 14: 868-874, incorporated by reference herein) observed that approximately 70% of patients suffering from an acute plaque rupture were initiated on plaques that were less than 50% occluded as revealed by previous coronary angiography. This and similar observations have been confirmed by other investigators (Nissen, S. Coronary angiography and intravascular ultrasound. Am J Cardiol 2001; 87 (suppl): 15A-20A, incorporated by reference herein).
The development of technologies to identify these unstable plaques holds the potential to decrease substantially the incidence of acute coronary syndromes that often lead to premature death. Unfortunately, no methods are currently available to the cardiologist that may be applied to specify which coronary plaques are vulnerable and thus prone to rupture. Although treadmill testing has been used for decades to identify patients at greater cardiovascular risk, this approach does not have the specificity to differentiate between stable and vulnerable plaques that are prone to rupture and frequently result in myocardial infarction. Inasmuch as a great deal of information exists regarding the pathology of unstable plaques (determined at autopsy) technologies based upon identifying the well described pathologic appearance of the vulnerable plaque offers a promising long term strategy to solve this problem.
The unstable plaque was first identified and characterized by pathologists in the early 1980's. Davis and coworkers noted that with the reconstruction of serial histological sections in patients with acute myocardial infarctions associated with death, a rupture or fissuring of atheromatous plaque was evident (Davis M J, Thomas A C. Plaque fissuring: the cause of acute myocardial infarction, sudden death, and crescendo angina. Br Heart J 1985; 53: 363-373, incorporated by reference herein). Ulcerated plaques were further characterized as having a thin fibrous cap, increased macrophages with decreased smooth muscle cells and an increased lipid core when compared to non-ulcerated atherosclerotic plaques in human aortas (Davis M J, Richardson P D, Woolf N, Katz D R, Mann J. Risk of thrombosis in human atherosclerotic plaques: role of extracellular lipid, macrophage, and smooth muscle cell content, incorporated by reference herein). Furthermore, no correlation in size of lipid pool and percent stenosis was observed when imaging by coronary angiography. In fact, most cardiologists agree that unstable plaques progress to more stenotic yet stable plaques through progression via rupture with the formation of a mural thrombus and plaque remodeling, but without complete luminal occlusion (Topol E J, Rabbaic R. Strategies to achieve coronary arterial plaque stabilization. Cardiovasc Res 1999; 41: 402-417, incorporated by reference herein). Neo-vascularization with intra-plaque hemorrhage may also play a role in this progression from small lesions (<50% occluded) to larger significant plaques. Yet, if the unique features of unstable plaque could be recognized by the cardiologist and then stabilized, a dramatic decrease may be realized in both acute myocardial infarction and unstable angina syndromes, and in the sudden progression of coronary artery disease.
The embodiments disclosed herein uses depth-resolved light reflection or Optical Coherence Tomography (OCT) to identify the pathological features that have been identified in the vulnerable plaque. In OCT, light from a broad band light source or tunable laser source is input into an interferometer with a portion of light directed to the vessel wall and the other portion directed to a reference surface. The distal end of the optical fiber is interfaced with a catheter for interrogation of the coronary artery during a heart catheterization procedure. The reflected light from the plaque is recombined with the signal from the reference surface forming interference fringes (measured by a photovoltaic detector) allowing precise depth-resolved imaging of the plaque on a micron scale.
OCT uses narrow linewidth tunable laser source or a superluminescent diode source emitting light over a broad bandwidth (distribution of wave length) to make in situ tomographic images with axial resolution of 10-20 μm and tissue penetration of 2-3 mm. OCT has the potential to image tissues at the level of a single cell. In fact, the inventors have recently utilized broader band width optical sources such as femto-second pulsed lasers, so that axial resolution is improved to 4 microns or less. With such resolution, OCT can be applied to visualize intimal caps, their thickness, and details of structure including fissures, the size and extent of the underlying lipid pool and the presence of inflammatory cells. Moreover, near infrared light sources used in OCT instrumentation can penetrate into heavily calcified tissue regions characteristic of advanced coronary artery disease. With cellular resolution, application of OCT may be used to identify other details of the vulnerable plaque such as infiltration of monocytes and macrophages. In short, application of OCT can provide detailed images of a pathologic specimen without cutting or disturbing the tissue.
One concern regarding application of this technology to image atherosclerotic plaques within the arterial lumen is the strong scattering of light due to the presence of red blood cells. Once a catheter system is positioned in a coronary artery, the blood flow between the OCT optical fiber and artery can obscure light penetration into the vessel wall. One proposed solution is the use of saline flushes. Saline use is limited in duration, however, since myocardial ischemia eventually occurs in the distal myocardium. The inventors have proposed the use of artificial hemoglobin in the place of saline. Artificial hemoglobin is non-particulate and therefore does not scatter light. Moreover, artificial hemoglobin is about to be approved by the United States Food and Drug Administration as a blood substitute and can carry oxygen necessary to prevent myocardial ischemia. Recently, the inventors demonstrated the viability of using artificial hemoglobin to reduce light scattering by blood in mouse myocardium coronary arteries (Villard J W, Feldman M D, Kim Jeehyun, Milner T E, Freeman G L. Use of a blood substitute to determine instantaneous murine right ventricular thickening with optical coherence tomography. Circulation 2002; Volume 105: Pages 1843-1849, incorporated by reference herein).
The first prototype of an OCT catheter to image coronary plaques has been built and is currently being tested by investigators in Boston at Harvard-MIT (Jang I K, Bouma B E, Kang D H, et al. Visualization of coronary atherosclerotic plaques in patients using optical coherence tomography: comparison with intravascular ultrasound. JACC 2002; 39: 604-609, incorporated by reference herein) in association with Light Lab Co. The prototype catheter consists of a single light source and is able to image over a 360 degree arc of a coronary arterial lumen by rotating a shaft that spins the optical fiber. Because the rotating shaft is housed outside of the body, the spinning rod in the catheter must rotate with uniform angular velocity so that the light can be focused for equal intervals of time on each angular segment of the coronary artery. Mechanical drag in the rotating shaft can produce significant distortion and artifacts in recorded OCT images of the coronary artery. Unfortunately, because the catheter will always be forced to make several bends between the entry point in the femoral artery to the coronary artery (e.g., the 180 degree turn around the aortic arch), uneven mechanical drag will result in OCT image artifacts. As the application of OCT is shifted from imaging gross anatomical structures of the coronary artery to its capability to image at the level of a single cell, non-uniform rotation of the single fiber OCT prototype will become an increasingly problematic source of distortion and image artifact.
Essentially, a current endoscope type single channel OCT system developed by Light Lab Co. suffers by non-constant rotating speed that forms irregular images of a vessel target. See U.S. Pat. No. 6,134,003, incorporated by reference herein. Their approach of a rotary shaft to spin a single mode fiber is prone to produce artifact. The catheter will always be forced to make several bends from its entry in the femoral artery, to the 180 degree turn around the aortic arch, to its final destination in the coronary artery. All these bends will cause uneven friction on the rotary shaft, and uneven time distribution of the light on the entire 360 degree arch of the coronary artery. As the application of OCT is shifted from gross anatomical structures of the coronary artery to its capability to image at the level of a single cell, then non-uniform rotation of the single fiber OCT will become even a greater source of greater artifact.
The embodiments disclosed herein solve rotational distortion and related artifactual problems by developing a multiphase array OCT catheter. By incorporating 10-60 individual OCT fibers within a single catheter, rotation of the optical fiber or similar element (e.g., micro-motor driven mirror) and associated image distortion and artifacts are eliminated and spatial resolution may be improved. The catheter will allow 10-60 individual sources of light to independently image the 360 degree arc of the coronary arterial lumen. An additional advantage of the multiphase array is provision of greater spatial resolution of the object being interrogated in comparison to single fiber designs. Many investigators recognize that a single rotating fiber or micro-motor driven mirrors utilized in current designs will not allow imaging at the level of a single cell while the multiphase array approach can provide cellular resolution.
The construction of a multiphase array OCT catheter requires resolution of a number of problems using innovative design solutions. Successful design and demonstration of the catheter requires the development of an optical channel containing 10-60 individual fibers in a 1.5 mm diameter. Each fiber requires a lens to focus the light, and a mirror fabricated using nanotechnology to redirect light from each fiber by 90 degrees from the catheter to the luminal surface of the coronary artery. Further, each of the 10-60 light paths has to be split again for both reference and artery paths. The embodiments disclosed herein provide design solutions to both the catheter and multichannel interferometer.